Metal-air electrochemical cells utilize oxygen from ambient air as a reactant in an electrochemical reaction to provide a relatively lightweight power supply. Generally described, a metal-air cell includes an air-permeable cathode and a metallic anode separated by an aqueous electrolyte. During operation of a zinc-air cell, for example, oxygen from the ambient air is converted at the cathode to hydroxide ions, zinc is oxidized at the anode and reacts with hydroxide ions, and water and electrons are released to provide electrical energy.
Cells that are useful for only a single discharge cycle are called primary cells, and cells that are rechargeable and useful for multiple discharge cycles are called secondary cells. An electrically rechargeable metal-air cell is recharged by applying voltage between the anode and the cathode of the cell and reversing the electrochemical reaction. During recharging, the cell discharges oxygen to the atmosphere through the air-permeable cathode and the anode is electrolytically reformed by reducing to the base metal, the metal oxides formed during discharge.
Metal-air cell anodes are made from metals which can be oxidized during discharge in a metal-air cell to produce electrical energy. Such metals include lead, zinc, iron, cadmium, aluminum, and magnesium. Zinc is normally preferred because of the availability, energy density, safety, and relatively low cost of zinc. One problem with using zinc as the anode in a metal-air cell, however, is that zinc tends to corrode in the cell and produce gas. Excessive gassing at the anode produces pressure within the anode and can cause the cell to rupture. Mercury is added to the zinc to alleviate the problem of gassing at the anode. In other words, the zinc is amalgamated. The addition of mercury increases the cycle life of the cell by reducing the gassing at the anode. A serious problem with mercury, however, is that mercury is very toxic.
Indium has been used as a non-toxic substitute for mercury in zinc anodes. Typically, indium is added to zinc to form an alloy and the alloy is melted and blown to form a zinc-indium powder. Alternatively, the zinc can be formed as a powder and then coated with indium to form an indium coated zinc powder. The zinc/indium powder is then used to form an anode by either mixing the powder with a gel to form a gel-type zinc anode or pressing and sintering the powder into a cake.
Zinc powder gel anodes made with zinc powder are effective and have been used successfully, but there are some drawbacks. For example, it is desirable to have an anode which is uniform in thickness, density and porosity. This is desirable so that the anode discharges uniformly and efficiently and remains conductive across substantially the entire anode surface. Areas of an anode which are thicker, more dense, or less porous will discharge more slowly and can become inactivated by passivation. Passivated areas have reduced conductivity and the recharge efficiency of the anode is diminished.
The particle size and particle size distribution of zinc powder particles affect the density and porosity of the anode. Zinc powder often has a non-uniform particle size distribution, and as a result, relatively fine zinc particles tend to collect together and form densified, low porosity areas. In addition, zinc powder anodes are typically made by combining the zinc powder with binder material, polymer fibers, and electrolyte, and pasting the slurry to form a cake or gel. Care must be taken to thoroughly mix these materials so that the concentration of each ingredient is uniform across the anode. Some ingredients such as polymer fibers, tend to collect together during mixing and result in an anode with non-uniform density and porosity. Furthermore, the powder anode is usually formed with a tool such as a doctor blade which has limited precision and often results in an anode of uneven thickness. As a result, zinc powder anodes are often non-uniform and have a limited cycle life.
Non-particulate metal plate zinc forms an anode which is more uniform in thickness, density, and content; however, zinc/indium alloy can not be formed into solid plate as the indium goes to the grain boundaries of the zinc and will not allow it to be extruded into solid plate. In addition, zinc plate which is not amalgamated passivates during the initial discharge in a metal-air cell, discharges at an inadequate current density, and suffers a permanent loss in zinc capacity. This problem is not as serious for zinc powder anodes because zinc powder anodes have a higher surface area than zinc plate. However, to prevent the gassing and corrosion problems, these zinc powder anodes, as discussed above, use mercury.
These anodes are generally made using an ultrasonic welding process wherein a zinc portion of the anode and a current collector, usually made from silver, are bonded through ultrasonic welds. However, better contact between the zinc and the silver is desired than is achieved with this method. Additionally, the welded spots are inactive, thereby reducing the capacity of the anode.
Accordingly, there remains a need for a mercury-free zinc anode with uniform thickness, density and porosity, reduced gassing and corrosion at the anode and enhanced cycle life. Additionally, there exists a need for a method of forming an electrode which creates good contact between the anode and the current collector while eliminating dead spots within the anode.